Introduction: Buserelin or gonadotropin-releasing hormone agonist (GnRH agonist) is used to control steroidal hormone secretion in infertility centers. The GnRH receptors have been found to be expressed in rat endometrium. Thus, the present study aimed at examining the effects of GnRH-agonist administration on endometrium and steroidal hormone secretion in immature rat. Methods: In this experimental intervention study, twelve 30-day-old Wistar rats were divided into two groups of experimental and control (n = 6). The study and control groups received 300 µg/kg GnRH agonist and normal saline, respectively, for 5 days subcutaneously. After 24 hours, the rats were anesthetized. Blood samples were obtained from the rats' hearts for the assessment of steroidal hormones and then uterus was removed. After tissue processing and sectioning, the sections were stained by hematoxylin and eosin and Periodic Acid-Schiff. Data were analyzed using MannWhitney U test and t test by the SPSS 20 software. P ≤ 0.05 was considered statistically significant. Results:The mean of endometrium thickness was increased from 899.27 ± 304.65 µm in the control group to 1240.49 ± 349.23 µm in the experimental group; this difference was statistically significant (P ≤ 0. 001). There was no statistically significant difference in the mean of epithelium height between the two groups. Also, the results showed no significant difference in 17 B-stradiol and progesteron plasma levels between the two groups. Conclusions:The findings of the present study suggest that short-term administration of Buserelin can increase thickness of endometrium in prepubertal female rats.